1. Field of the Invention
The invention relates to a liquid crystal display device having, between a polarizer and an analyzer, a first layer of twisted liquid crystal material with a twisted structure between two transparent substrates, with pixels being realized between the substrates, and a compensator layer. Such display devices are generally used in, for example, automotive displays, but also in monitors, etc.
2. Description of the Prior Art
An example of such a display device is shown in, for example, U.S. Pat. No. 5,287,207. This patent shows a double cell in which a second cell functions as a compensator in order to obtain an optimum contrast. Although a satisfactory contrast is obtained in these types of display devices upon perpendicular passage of the light, it appears that there is a considerable grey scale inversion in such a double cell, when viewed at an angle.
It is, inter alia, an object of the present invention to reduce or eliminate said grey scale inversion.
To this end, a display device according to the invention is characterized in that a pixel comprises at least two sub-pixels having the same twist and, viewed transversely to the substrates, twist angles which are rotated with respect to each other. The twist angles, viewed transversely to the substrates, are rotated substantially 180 degrees with respect to each other. The effects on the grey scale of one sub-pixel are then compensated, as it were, by the effects of the other sub-pixel. Also in the case of rotations different from 180 degrees, an enhancement is obtained dependent on the type of usage (for example, in cockpit applications or in applications where a display device is viewed by different persons from two directions).
The twist angles are preferably in the range between 50 and 100 degrees.
A first preferred embodiment of a liquid crystal display device according to the invention is characterized in that the compensator layer has a twisted structure with a twist which is opposite to that of the layer of twisted liquid crystal material. The compensator layer preferably has a twist angle which is opposite to that of the layer of twisted liquid crystal material.
Further enhancements are possible by adding a retardation layer with an optical axis perpendicular to the compensator layer.
It is also possible to use negative birefringent materials such as discotic materials for the compensator layer.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.